We don't have three different axes for the warm colors the way we do for the cool ones.
Our trichromancy would be improved if those three peaks were more evenly distributed. This happens in non-mammalian trichromat vision.
There's a bit of fudge in that graph.
It doesn't represent the actual pigment response in the cones to those frequencies. The short range is filtered out by the lens.
If you replace your lens with an artificial one that lacks a UV filter, suddenly you get responses deep into the 300s. The world changes. You also get retinal cancer if you don't take constant protective measures.
This was discovered because the earliest cataract surgeries didn't know to use a UV filter.
Human tetrachromats aren't like non-mammalian tetrachromats because you have two variant Xs that express a slightly different peak sensitivity to the medium cones.
And X-deactivation guarantees that both will be active in the same retina.
The peaks of the two duelling Xs are too close together to really think of it as tetrachromacy in the way it would be in say a bird or an insect.
Then again, they probably say the same about our trichromats, too. :)
Dinosaurs are very judgemental.
Human tetrachromats must be female, and they can only happen due to a mutation in one or both Xs.
I leave it to you to deduce why some simians are trichromats only in the female.
Same reason you aren't "supposed" to have male calico cats: not enough Xs.
"Am I an orange kitty" and "Am I a black kitty" is a gene on the X chromosome.
So you can't get two different answers without two different Xs.
However, it only takes a Y to make a mammal male. It doesn't matter how many Xs they have. So male calicos are invariably Klinefelters or more complex mosaics.
@terdon This article may have bearing on the Biology question I edited:
The evolution of color vision in primates is unique compared to most eutherian mammals. A remote vertebrate ancestor of primates possessed tetrachromacy, but nocturnal, warm-blooded, mammalian ancestors lost two of four cones in the retina at the time of dinosaurs. Most teleost fish, reptiles and birds are therefore tetrachromatic while all mammals, with the exception of some primates and marsupials, are strictly dichromats.
Primates achieve trichromacy through color photoreceptors (cone cells), with spectral peaks in the violet (short wave, S), green (middle wave, M), and yellow-green (long wave...
At variance with what that article says at its top, there is evidence that some felines may be trichromats.
> Many cat retinal ganglion cells (types X, Y, and W) have inputs from three separate cone systems. Those with peak sensitivities at 450 and 555 nanometers have been previously shown. A gamma max cone with a peak sensitivity of 500 nanometers can be differentiated from other cones by spectral sensitivity and from rods by receptive field differences, functioning above rod saturation levels, and by cone-rod breaks in the dark-adaptation curves.
The similarity of the three-cone cat retina to the extramacular retina of the rhesus monkeys suggests that the cat may have photopic trichromatic vision.
The feline vision system has significant differences from the human one, so it’s difficult to compare.
For one thing, their short cones peak lower than ours to, allowing them to see into the near UV. This gives them a different separation.
Another important difference is that their scotopic vision is much better than ours, in part because of having "more" rods, but more because of having a tapetum lucidum that we lack.
The big problem is that we can't easily ask a cat whether two oranges are the same, or two blues for that matter.
Cats don't have to "look away" from dim lights to see them the way we have to.
Because the centermost part of their retinas actually has rods in it; ours does not.
No foveola, nor even fovea IIRC.
So astronomers train themselves to look away from dim stars if they want to see them. Cats don't have to do that.
I know we don't have much in the way of short ("blue") cones there either. I don't know what this means for us.
I also wonder whether this might somehow be protective. Short wavelengths are higher energy.
"Color" isn't "real". It's all in the mind.
You can't even do a good job at modelling our perception of color without taking into consideration the dual-opponent cells.
This allows two instances of an absolutely identical RGB spectral mix to be perceived as being different "colors" because of their contrasting context. It also allows two instances of completely different RGB mixes to be perceived as being identical "colors" through metamerism. It's all very complicated.